1. Field of the Invention
This invention relates generally to cryogenic pump applications. More specifically, the invention relates to a seal assembly for cryogenic pump applications.
2. Description of the Background Art
In cryogenic applications, cryogenic materials are pumped from a source to the point of application. The nature of the materials used in cryogenic applications require specialized instrumentality. For example, the pumps that move the cryogenic product from its source to the point of application are highly specialized as are the seal assemblies involved in the delivery pathway. A seal assembly for cryogenic applications must keep atmospheric contaminants out of the cryogenic material, while keeping the cryogenic materials inside the source to application pathway. To meet the specialized needs of cryogenic sealing, standard bearing shaft seals must compensate for the extraordinary contraction of materials immersed in the cryogenic environment. Presently, current practice dictates a multi-piece sealing assembly consisting of a bellows, a loose shim stack determined empirically for each assembly, a rotating shaft seal ring, and a seal body.
A critical factor in the current practice for installing a seal assembly is providing a seal assembly with a seal compression within the range specified by cryogenic equipment manufacturers. A skilled artisan readily recognizes that manufacturers of cryogenic equipment to which seal assemblies are attached specify seal compression ranges. A seal assembly with a seal compression outside of the range will not operate properly. When installing a seal on a cryogenic pump, seal compression is determined by 1) measuring the and recording the distance from the end of the shaft to the rotating seal without compressing the bellows (free length state); 2) depressing the bellows/rotating seal combo and measuring the distance from the end of the shaft to the rotating seal (compressed state); 3) Calculating the difference between the free length state and the compressed state. If the seal compression is higher than the manufacturer""s recommended range, shims must be added to increase the compressed state measurement, which results in a lower seal compression. The following equation is illustrative:
Free Length Statexe2x88x92(Compressed State+X)=Seal Compression
In the above equation, the distance obtained by adding shims is represented as X.
Further illustrative of the current practice is the following example of the steps to install a seal assembly in a cryogenic pump application. The steps are as follows:
1. A dial indicator or caliper having the capability of measuring within 0.010 to 0.050 inch is mounted in position against the rotating seal on the shaft as shown in FIG. 1.
2. The dial indicator is set to read zero.
3. The impeller screw is then slowly loosened until the dial indicator reading stops increasing. This reading is the seal compression.
4. The reading is then confirmed several times by repeating steps 2 and 3, above.
5. If the reading is greater than the seal manufacturer""s specification, the impeller screw, dummy impeller, and rotating seal face are removed from the shaft and a second aluminum shim is inserted.
6. The procedure is again repeated to determine the new total seal compression.
7. If necessary, an additional seal is inserted. Generally, a maximum of six shims may be installed. If proper seal compression cannot be obtained with six shims, the shaft seal and O ring behind the flange are removed and another complete assembly must be installed on the shaft. The measurement procedure must then be repeated with the new assembly components in place. If correct seal compression still cannot be obtained, a shaft extension may be required.
This standard practice for installing the multi-part seal presents the following disadvantages:
1. Dimensional variations occur resulting in a varying amount of shims necessary to achieve proper seal compression;
2. As a consequence of the dimensional variations, no means of retaining the components within the seal body has been developed. Therefore, up until now and under the current practice, a single-piece universal seal was not possible;
3. The current sealing practice makes the assembly of the shaft seal time-consuming due to the labor-intensive nature of taking careful height measurements and selection of the suitable shim stack;
4. The labor intensive nature of the current practice increase the probability that the seal components will become contaminated from handling; and
5. There is an increased risk of human error.
It is therefore an object of the present invention to provide a single-piece universal seal to be used in cryogenic sealing. The present invention achieves this by controlling dimensional variation, for example by controlling bellows height which in turn provides for a single shim stack height thereby eliminating the high amount of variation which exists in current practice. It is another object of the present invention to reduce the current high amount of time and labor spent on shaft seal assembly by providing a single-piece seal assembly. It is still another object of the present innovation to reduce the likelihood of shaft assembly contamination, which is also achieved by providing a single-piece universal seal assembly.
The invention is a single-piece seal assembly used to prevent escape of cryogenic materials in cryogenic pump applications. The single-piece seal assembly also is effective in keeping atmospheric contaminants out. The single-piece seal assembly reduces the installation time and reduces the chances of contamination and human error.
The single-piece seal assembly comprises an annular seal body. A retainer is positioned at a first end of the seal body. A rotable seal is concentric with the retainer and has a notch positioned on an inner diameter of a first face of said rotable seal. In the notch is positioned a single shim. The shim is of a length such that it is capable of providing a selected seal compression. An annular bellows having first and second ends is affixed to an inner diameter of the seal body. On the first end of the bellows is a ring holder. The ring holder holds a ring seal.
To manufacture the single-piece seal assembly (hereinafter referred to as xe2x80x9cseal-assemblyxe2x80x9d), discrete sealing components are inserted into a seal body followed by a retainer which is fit onto the seal body to secure the components together, thereby forming the completed seal assembly. The bellows having a preset height provides the dimensional stability needed for the single-piece design. This bellows produces a stable and consistently accurate seal height, eliminating the need for a dial indicator measurement to determine the shim dimension for each installation, which as mentioned above, is currently the practice. The seal assembly includes a single shim stack equivalent (xe2x80x9cshimxe2x80x9d), which is adapted to provide a compressed state that will obtain a seal compression within the manufacturer""s specified range. The shim is fitted into a machined notch in the rotating seal. The retainer secures all seal components within the seal body to create the single-piece seal assembly.